The given sentence is in the positive degree, it speaks about the manner in which the person 'she' performs at the piano. From the options, 'well' is the most suitable option, it means 'in a good or satisfactory manner' and it's in the positive degree, hence option C is correct. 'Better than' is in the comparative degree, whereas the sentence is in the positive degree. A sentence in the comparative degree expresses a comparison between two people, or things where one is higher in degree or quality than the other. Here, there's only one person spoken about, hence option A is incorrect. Similarly, 'worse' is an adverb in the comparative degree, it means 'in a more disadvantageous way', and the sentence is not comparative, hence option D is also incorrect. 'Best than' is grammatically incorrect, 'best' is in the superlative degree and 'the' precedes every superlative adverb, 'than' is not used. 'Than' is in the comparative degree which compares two individuals, hence the phrase is wrong, making option B incorrect.